This invention relates in general to materials and processes for gas separation, and in particular to polymer nanocomposites for separating a target gas from a second gas in a gas mixture. For example, the polymer nanocomposites can be useful for separating carbon dioxide from the hydrocarbons present in natural gas.
Natural gas is important as a fuel and as a raw material in the petrochemical and other chemical process industries. The term “natural gas” generally refers to gaseous hydrocarbons (comprised of methane and light hydrocarbons such as ethane, propane, butane, and the like) which are found in deposits in the earth, often together with oil or coal. In common usage, deposits that are rich in natural gas are called natural gas fields and deposits that are rich in oil are called oil fields. Often, non-combustible gases such as carbon dioxide (CO2) and nitrogen, and contaminants such as hydrogen sulfide, are found in combination with the hydrocarbons.
The composition of natural gas extracted from the earth varies. For example, a high quality gas field may contain as much as 95% methane with only minor amounts of other materials. On the other hand, gas fields with high proportions of non-combustible gases and/or contaminants are common. For example, a CO2-rich gas field may contain 40% to 70% or more carbon dioxide. Also, natural gas extracted from an oil field using CO2 flooding for enhanced oil recovery may be high in carbon dioxide.
For most markets, it is desirable to minimize the presence of non-combustible gases and contaminants in the product gas. For example, a typical gas pipeline specification requires no more than 2% carbon dioxide. Before CO2-rich natural gas is sent to the supply pipeline the carbon dioxide content is reduced.
Various techniques are known for removing carbon dioxide from natural gas. Among the techniques is absorption of carbon dioxide with a chemical or physical solvent or with a molecular sieve. For example, absorption with an amine is a widespread commercial process.
Another technique for removing carbon dioxide from natural gas is separation with a polymeric gas separation membrane. Different types of polymer membranes allow different gases under pressure to pass through them faster than other gases. For example, membranes made from cellulose acetate or polyimide are more permeable to carbon dioxide than hydrocarbon gases. Given a feed stream of natural gas, the carbon dioxide will pass through or permeate across a cellulose acetate membrane faster than will the hydrocarbon gases, resulting in a permeate stream concentrated in carbon dioxide and a residue stream concentrated in hydrocarbon gases.
Preferably, a polymeric gas separation membrane has a high selectivity or separation factor, i.e. the membrane is highly effective in separating carbon dioxide from hydrocarbon gases because it allows one type of gas to pass through much faster than the other. Also preferably, a polymeric gas separation membrane has a high flux, i.e., it allows a relatively high rate of flow of gas through the membrane so that the separation process is useful on a commercial scale. However, membranes with high selectivity are generally characterized by low permeability, while membranes with high permeability generally have low selectivity.
Another problem with polymeric gas separation membranes is that the gases can cause “fouling” or “poisoning” by plasticization or swelling of the membrane after a period of use, which damages the properties of the membrane and reduces its separation ability and flux. Also, some polymeric membranes lack sufficient strength and durability, or sufficient thermal and chemical stability, for use in a natural gas separation process. Lack of adequate strength leads to the use of thicker membranes, hindering the flow and transport of separated gases.
Different polymeric gas separation membranes and membrane composites have been tried in the past for separating carbon dioxide from hydrocarbon gases. For example, membranes have been made from polymers with heterocyclic functionality on the backbone to improve the selectivity of the membrane. Polymer crosslinking or processing technology has been used to improve the flux of the membrane. The separation membranes are often relatively thick to provide sufficient strength and durability, but the increased thickness adds to the resistance to gaseous flow and thus decreases flux.
Previously known polymeric membranes for separating carbon dioxide from hydrocarbon gas have suffered from various drawbacks and have not been totally successful. Consequently, it would be desirable to provide improved polymeric compositions for gas separation processes such as separating carbon dioxide from hydrocarbon gas.